i am looking for a choice of buffer op-amp for using as inputto an adc converter the voltage will not go above 2 voltsand will not have any fast changes it will be adjusted byhand so we are not talking even 10Hz i guess.

not so familiar with using them in ad conversion

thanx in advance

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i am looking for a choice of buffer op-amp for using as inputto an adc converter the voltage will not go above 2 voltsand will not have any fast changes it will be adjusted byhand so we are not talking even 10Hz i guess.

not so familiar with using them in ad conversion

thanx in advance

Depending on the A/D sometimes these have an anti-alias filter in front of them that the opamp has to drive.

i choose a MCP3424 since it was supported in ccs and i got it working in proteus. so i can makea choice between 12-18 bits my choice is not definitive so far. i need to get the pic and the adcfor practical testing so i can see in real life how the circuit behaves.

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the more you learn, the lesser you get to understand. is it then good know alot but not understand what you know or.......

i choose a MCP3424 since it was supported in ccs and i got it working in proteus. so i can makea choice between 12-18 bits my choice is not definitive so far. i need to get the pic and the adcfor practical testing so i can see in real life how the circuit behaves.

Ok, sounds ok, but I do circuit design based on performance requirements and understanding the application needs. Picking ADC because its supported in CCS seems like it is not very important to you so you are just playing around with no requirement. Anything will work for this purpose and you don't need expert help.

@sphinx, if you are looking to use the entire scale of a 18bit ADC and your max amplitude of the signal is 2V, meaning that you are going to use a 2V voltage reference, then you will have about 7uV per step which it's a challenge even to design the PCB itself. You'll have a lot of noise and it will require some hard engineering where the choice of opamp buffer is only the beginning.

But if you don't need performance, and the signal variation is small, then you could do some software oversampling and then dumping the lower digits (where the noise should concentrate) to arrive to the original 18bit result.

And as a medium range opamp buffer, I would choose one that is recommended in the datasheet as a buffer for ADC. One like, OPA2350 http://www.ti.com/lit/ds/symlink/opa2350.pdf but you'll have to deal somehow with the about 150uV offset.

thanks for inputs i will try not to use any buffert op-amp and i am not going to use 18 bit but i will try with 16bitwhich in reality will be 15bit since adc has a swing -2.048 - 0 - 2.048 but i am only using the positivie half.

if this does not work i will try lower bitrates i could not find a 0-5v swing on an adc that i could get to workwith ccs with 12-16 bit to try software out.

@ optikon. no this is not for any design its just a private project i am working on. i do these since whiplash todo some things at least to use my head and not stagnate and stop learning. it was over 10-13 years ago idid any work at all so i do these small projects to do just about to have a bit of fun and to try a bit harderto learn new things since i like to learn and experience i am learning altium with more and more featureseach time i try it i am not in any way any expert on it but its fun to use and learn something new, just to andstay sharp, and i appreciate all your input it also teaches me something new and things i didn't think of.

i just picked an adc in ccs since i dont want to write libs and include files to get an adc to work withwith a compiler thats a bit of over my head work. i started with arduino and i then i wanted to try C so tried ccs c for pics and found a pic fairly cheap and got an adc to play with i will attach for those thatwant have a peek at it. the software needs some tuning i am not satisfied with some parts of it, its awork in progress so far.

many thanxsphinx

« Last Edit: July 06, 2017, 03:23:51 03:23 by sphinx »

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I would not use an op-amp buffer this will introduce unwanted noise, I've done a lot of playing with this one and the MCP3426 version, below is the circuit I use where a add a filter to filter out about 10hz the readings become very stable and actuate. ignore the the op-amp part as they boost the voltage from a shunt which can measure +/- signals, could be used for voltage converter just change the values around and calculate for voltage, I also used this in a wireless meter which can measure voltage +/100v. Using a 2.048 vref the output voltage reads half of vref then either adds or subtracts the voltage from vref, if you just wanted to measure 2.00v then just try the filter the. Just use the 2 resistors and capictors I used a 1uf and 10uF ceramic caps.

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We do a lot of op-amp circuits driving ADCs at my work. A favorite part is the OPA2333 from TI.

One thing to watch out for, with any op-amp, is to answer the question, "does the output of the op-amp need to go down to it's bottom power supply rail?" For example, are you powering the op-amp from 0V & 3.3V and you need the op-amp to be able to output a voltage down to 0V. For single-supply circuits this is a common consideration. If you need the op-amp to be accurate down to within a few mV of it's bottom supply rail (0V in my example) then that limits your choice of op-amps. Linear Technology is a good place to look for op-amps that can do that.

The OPA2333 will accurately output a voltage down to within 50mV (less actually) of its lower rail.

It is worth to note, that most of the specification is valid for signals around zeroand bipolar supply and can extremely degrade if the rail-to-rail feature is abused.For maximum (DC-)precision at near zero levels i would always recommend a bipolar supply.